Chapter 33 Control Systems in Plants
1) Plant Hormones
a) Experiments on how plants turn toward light led to the
discovery of plant hormones
i)
phototropism
(1) growth of a shoot towards light
(2) cellular mechanism
(a)cells on dark side of plant are larger
(i) elongated faster
(ii) different growth rate makes shoot bend
(b) what causes different rate of growth?
(i) hormones
ii) 1880 – Charles Darwin and son Francis
(1) observation – grass seedling grow toward light only if
shoot tips were present
(2) Figure 33.1C
(3) conclusion – tip of root responsible for sensing light
(4) observation – bending occurred below tip
(a)signal must be sent from tip
iii) 1913 Peter Boysen-Jensen – Danish botanist
(1) last two plants in Fig. 33.1C
(a)gelatin block allowed signals to pass, but cells not in
contact
(b) mica is impermeable
(2) conclusion – signal is a mobile chemical
iv) 1926 Fritz Went – Dutch Grad Student
(1) extracted the chemical messenger (Fig. 33.1D)
(a)cut off tip of grass
(b) put on agar block
(c)chemical should diffuse into agar
(d) put agar block on plant
centered, grew straight up
1. grew faster than controls
2. concluded chemical stimulated it to grow
(ii) placed block off-center
1. blades bent away from side with chemical
block
2. concluded higher growth promoting chemical
on darker side
3. Went termed the chemical “auxin” (auxein = to
increase (Greek))
(iii)studies with other plants do not support this
though
1. greater conc. of chemical on lighted side of
stem in these cases (sunflowers, radishes, other
dicots)
2. not all plants work the same way
b) Five major types of hormones regulate plant growth and
development
i)
Listed in table on page 662
(1) affect growth and development by affecting division,
elongation and differentiation of cells
(2) trigger signal transduction pathways as usual
(a)turn on and off genes
(b) inhibit or activate cellular enzymes
(c)changes in membrane properties
(3) all 5 influence growth
(4) 4 influence differentiation
(5) effect of hormone depends on plant species, site of
action in plant, concentration, developmental stage of
plant
(6) hormones rarely act alone, it is a combination of
hormones and their concentrations
c) auxins stimulate elongation of cells in young shoots
i)
auxin
(1) any chemical that promotes seedling elongation
(i)
(2) indoleacetic acid (IAA) – major auxin in plants
(3) Figure 33.3A
(a)taller
pea seedling got auxin (IAA)
(4) evidence suggests it is produced by apical meristem at
tip of shoot
(a)moves down and promotes growth of stem by causing
cells to elongate
(b) depends on concentration of auxin though (Fig
33.3B)
(i) above .9g/L it inhibits elongation
(ii) high levels causes cells to secrete ethylene
1. counters effects of auxin
(c)Figure 33.3B – bottom line
(i) same hormone, different effect on different tissues
at different concentrations
(5) how does auxin cause them to elongate?
(a)hypothesis: weakening cell walls
(i) stimulate proteins to pump protons into cells wall,
which activate enzymes that break H-bonds crosslinking cellulose
(ii) cell swells with water
(iii)auxin stimulates cell to make more cell wall
material to sustain size
(6) other effects of auxin
(a)induces cell division in vascular cambium
(i) promote growth in diameter
(b) produced by developing seeds
(i) promotes fruit growth
(ii) some plants develop fruit w/o fertilization if
sprayed with auxin
1. seedless tomatoes, cucumbers, eggplants
d) cytokinins stimulate cell division
i)
cytokinins
(1) growth regulators that promote cell division (named
after cytokinesis)
(2) made in roots mostly
(3) retard aging of flowers and leaves
(a)florists
spray plants with cytokinins
(4) effects of cytokinins are influence by concentration of
auxins
(a)Figure 33.4
(i) auxins from terminal bud move down plant inhibit
axillary buds
1. grow more in height
(ii) no terminal bud
1. cytokinins transported up from roots activated
axillary buds since auxins not around to inhibit
a. use cytokinins on christmas trees
(5) result
(a)lower axillary buds tend to grow before ones closer to
terminal bud
(b) perhaps, as roots become more numerous, cytokinin
concentrations increase and outcompete auxin levels
instructing plant to grow more branches, which roots
can now support
(c)antagonistic relationship (common among plant
hormones)
e) Gibberellins affect stem elongation and have numerous other
effects
i)
Fungus – genus Gibberella
(1) rice (Fig. 33.5A) - infects seedlings – grow taller, will
topple over and die before flowers produced
(a)Japan – call it “foolish seedling disease”
(i) Asia has suffered crop losses for centuries
(2) Japanese scientists (1920’s)
(a)discovered chemical released by fungus that causes
disease
(i) gibberellin
1. later discovered it exists in plants naturally
2. disease caused by an overdoes of gibberellin
ii)
gibberellins
(1) more than 100 known
(2) produced in roots and young leaves
(3) stim. elongation and division of cells in stems
(a)enhances auxin
(4) influence fruit development
(a)giberellin-auxin sprays to grow fruit without
fertilization (apples, currants, eggplants)
(b) Thompson seedless grapes (green grapes)
(i) makes grapes grow wider and farther apart (Fig.
33.5B)
(5) important in seed germination
(a)some seeds need special environmental conditions to
germinate (exposure to light, cold temp., etc…)
(i) spray with gibberellins and they germinate
regardless
(ii) likely link environmental cues with metabolism in
seeds
1. water becomes available, embryo releases
gibberellin, seed germinates
f) Abscisic acid (ABA) inhibits many plant processes
i)
Two teams
(1) one studying bud dormancy
(2) one studying leaf abscission (dropping of leaves)
ii) Both teams discovered ABA
(1) not really involved much in either ironically
(2) generally slows down growth
(a)seed dormancy
(i) important, don’t want seed in autumn to
germinate and be killed by winter
(ii) why don’t they germinate within fruits?
1. ABA
iii) ABA must be inactivated for germination to occur in
many seeds
(1) cold temps. trigger this in some seeds
(a)winter
necessary for spring germination!
(2) deserts – some seeds lay dormant until heavy rains
(a)wash ABA out of seed! (Fig. 33.6)
iv) gibberellins promote germination
(1) ratio of gibberellins to ABA determines germination for
many seeds
v)
ABA is internal signal for plants to withstand drought
(1) plant wilts
(2) ABA accumulates in leaves
(3) signals stomata to close (pump out K+)
g) Ethylene (it’s a gas) triggers fruit ripening and other aging
processes
i)
Early 1900’s
(1) fruit ripened in sheds with kerosene stoves
(a)thought is was heat doing this
(2) replace kerosene with cleaner-burning stoves
(a)fruit did not ripen as quickly!
ii) ripening was due to ethylene
(1) gaseous by-product of kerosene burning
(2) plants produce own ethylene
(a)triggers aging processes
(i) fruit ripening
(ii) apoptosis (programmed cell death)
(iii)others
(3) also produced in response to stress
(a)drought, flooding, mechanical pressure, injury, and
infection
iii) fruit ripening
(1) burst in fruit triggers ripening
(2) signal spreads from fruit to fruit (it’s a gas)
(a)one bad apple does spoil the lot
(3) enzymatic breakdown of cell walls
(a)softens fruit
(4) conversion of starch to sugar
(a)makes fruit sweet
(5) new scent and colors attract animals, eat fruit, disperse
seeds with a nice fertilizer
(6) put fruit in plastic bag to ripen quicker
(7) Figure 33.7A
(8) many fruits are picked green (tomatoes) and partially
ripened in huge bins with ethylene pumped over them
(9) to slow ripening
(a)apples flushed with CO2, which inhibits action of
ethylene
(b) circulate gas around apples to prevent accumulation
of ethylene
(i) pick apples in autumn and sell following summer
iv) falling of leaves
(1) color change
(a)leaves lose chlorophyll
(b) new pigments made and pigments already present,
but masked by chlorophyll
(2) before leaves fall
(a)essential elements are removed and stored in stem
(3) base of leaf stalk (petiole) separates from stem
(a)abscission layer
(i) separation region (Figure 33.7B)
(ii) narrow band of parenchyma cells with thin walls
1. walls further weakened by enzymes
(iii)abscission layer split when leaf falls
(iv) protective layer – forms scar on stem before leaf
falls
1. dead cells cover scar – prevent infection
(4) triggered by environmental stimuli
(a)shortening days
(b) cooler temps – lesser extent
(5) stimuli cause a change in balance of auxin and ethylene
(a)auxin, made in leaf, prevents abscission and promotes
metabolism
(b) as leaf ages, less and less auxin is produced
(c)ethylene
is produced more and more forming
abscission layer
(i) promotes synthesis of enzymes to break cell walls
of layer
(6) prevents drying out and frost damage
h) Plant hormones have many agricultural uses
i)
synthetic auxins
(1) prevent oranges and grapefruit trees from dropping
fruits before they can be picked (Fig. 33.8)
ii) large doses of auxins
(1) promote premature fruit drop
(2) spray on apple and olive trees to thin fruits
(a)fruits remaining will grow larger
(3) ethylene
(a)thins peaches and prunes
(b) loosens hold of berries, grapes and cherries for
machine picking
iii) large dose of gibberellins
(1) promote seed production in year one of carrots, beets
and cabbage (biennial plants)
2) Growth responses and biological rhythms in plants
a) Tropisms orient plant growth toward or away from
environmental stimuli
i)
tropisms
(1) directed growth responses that cause parts of a plant to
grow towards or away from a stimuli
(a)phototropism
(i) response to light
1. grasses
a. shoot tip contains pigment that somehow
senses the light and passes the message to
molecules that affect auxin transport
(b) gravitropism
(i) response to gravity (Fig. 33.9A)
hypothesis
a. gravity pulls dense organelles with starch
granules to low points of cells
b. send signal to cell to redistribute auxin
(ii) bottom line
1. doesn’t matter how seed lands, roots will grow
down and shoots up
(c)thigmotropism
(i) growth movement in response to touch
(ii) thigma = touch in Greek
(iii)tendril of pea plant (example)
1. grows straight until touches a support
2. contact causes cells to grow at different rates
on opposite sides of tendrils
a. slower in contact area
b. result: tendril coils around branch
(iv) allows plants to use other objects for support why
heading for sun (phototropism)
1.